Volume 28, Number 3, February 2016
Activity
The heart and circulation
Martin Rowland
This is a large topic that is common to all GCE AS and A-level specifications as well as to the
International Baccalaureate. This makes it a topic that is well worth revising, ensuring that you develop
all the skills that might be tested in your examination.
In the following exercise:

questions 2 and 3 cover subject content from the OCR, Edexcel and WJEC Eduqas
specifications, but not the AQA specification. Students following the AQA specification should,
however, be able to use their understanding of blood flow to suggest a correct explanation (an
AO2 skill) in question 3.

the final two sentences in question 13 cover subject content that is in the second year of
A-level study but not in the first year/AS content.
Revision tip: Always check the subject content of the specification you are following. This tells you
the concepts, facts, principles and terminology that examiners expect you to recall with understanding
(AO1 skills).
Try the following questions without using your notes or your textbook. You could do this alone or by
discussion in pairs.
Questions
1 Not all animals have a circulatory system. Explain the biological advantage of a circulatory system in
those animals that do.
2 There are different types of circulatory system.
(a) Describe the difference between a closed circulatory system and an open circulatory system.
(b) Insects have an open circulatory system yet remain active. Explain how their respiratory system
overcomes the disadvantage of having an open circulatory system.
3 Figures 3 and 4 of Dr Shiels’ article (pages 2–6 of the magazine) represent the circulatory system of
a mammal and of a fish. Which is the more efficient? Use evidence from Figures 3 and 4 to explain
your answer.
4 Global warming affects the temperature of lakes and oceans, as well as the atmosphere. Other than
any effect on enzyme activity, why might global warming affect the fish populations in the waters
around the UK?
5 Copy and complete the following table to compare the structure of mammalian blood vessels. State
whether each feature is present or absent and for the first three features state whether they are thick
or thin if present.
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Feature
Artery
Capillary
Vein
Protective outer
fibrous layer
Muscle layer
Elastic tissue layer
Semi-lunar valves
along whole length
Single layer of
squamous cells
forming endothelium
6 Figure 1 in Dr Shiels’ article shows a vertical section through a mammalian heart. Use information
from Figure 1 to explain why the pressure of blood leaving the left ventricle is higher than that leaving
the right ventricle.
7 Blood flow through the aorta is pulsatile but becomes less pulsatile as blood flows through the
circulatory system. It is no longer pulsatile in the vena cava. Explain why blood flow loses its pulsatile
nature as it passes through the mammalian circulatory system.
8 The graph represents the pressure changes in the left atrium, left ventricle and aorta during one
complete cardiac cycle of a human.
(a) Calculate the heart rate represented in this graph.
(b) P and Q represent times at which a change occurs in one or more valves within the heart. Use
information from the graph to describe and explain what happens at P and at Q.
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9 Dr Shiels tells us that Chris Hoy has a resting heart rate of 30 beats per minute. Suggest why this
does not lead to a reduction in the volume of blood flowing around his circulatory system.
10 Explain the term myogenic.
11 Impulses generated by the SA node can pass to the ventricles only through the AV node, where
they are delayed slightly (see Figure 2.1 on page 5 of the magazine). What is the advantage of this
delay?
12 The Purkyne fibres stimulate contraction of the ventricles from the base upwards. Explain the
advantage of this.
13 Complete the following passage by inserting the correct term in each blank space.
The cardiac cycle originates at the _________________, situated in the wall of the _______ atrium.
From here, impulses pass through the walls of the atria to the ________________ and from there
through the septum in the _________________________________ and up the walls of the ventricles
via the _____________________________. Although originating in the heart itself, the rate of the
cardiac cycle can be changed by the _______________________ nervous system. Release of
____________________________ by the _________________ branch causes an increase in heart
rate. Release of __________________________ by the ___________________________ branch
causes a decrease in the heart rate.
14 Devise a mark scheme of 6 marks for the following free-response question:
‘Explain how tissue fluid is produced.’
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Outline answers
1 A circulatory system enables mass flow, i.e. metabolites and waste products can be transported in
bulk in solution to and from exchange surfaces. This mass flow is faster than diffusion, enabling
animals to grow larger than they otherwise could.
2 (a) An open circulatory system lacks blood vessels whereas the blood is circulated within blood
vessels in a closed circulatory system.
(b) The small tracheoles run to cells in the tissues throughout an insect’s body. This allows diffusion
directly between the atmosphere and an insect’s body cells. It does, however limit the size to which
insects can grow, since diffusion is relatively slow over large distances.
3 The mammal’s system is more efficient. It is a double circulation, i.e. the circulation to the gas
exchange surface is separate from that to the rest of the body. There is a loss of pressure as blood
flows through blood vessels, particularly as it flows through smaller arterioles and capillaries. In a
single circulatory system, this loss of pressure occurs twice, once in the capillary beds in the gills and
again in the capillary beds of the body tissues. These losses result in very low blood pressure and,
hence, relatively slow circulation. While such slow circulation would not support the high metabolic
rate of mammals, it can support the lower metabolic rate of fish.
4 As water temperature rises, the solubility of gases in water becomes less (think of the bubbles as air
leaves solution as you boil a kettle of water). Oxygen concentration is an important abiotic factor
affecting fish populations. If the oxygen concentration falls as a result of warmer water, fish with a high
oxygen demand might not survive and could be replaced by a population of fish with a lower oxygen
demand.
5
Feature
Artery
Capillary
Vein
Protective outer
fibrous layer
Present; thick
Absent
Present; thin
Muscle layer
Present; thick
Absent
Present; thin
Elastic tissue layer
Present; thick
Absent
Present; thin
Semi-lunar valves
along whole length
Absent
Absent
Present
Single layer of
squamous cells
forming endothelium
Present
Present
Present
6 Figure 1 shows that the left ventricle has a much thicker muscle wall than does the right ventricle.
This means that it generates a greater force when it contracts than does the right ventricle.
Exam tip: In answers to a question like 6, students often tell examiners that the reason for the
difference in pressure is because the left ventricle ‘has got to pump blood all the way around the body
but the right ventricle only has to pump blood to the lungs’. This answer suggests that the heart knows
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what it is doing and has made a decision about the pressures it generates. This is, of course,
nonsense and this answer would gain no marks.
7 As blood flows through blood vessels, its pressure falls. This fall in pressure results from:

friction against the inner walls of blood vessels. The total surface area inside the blood vessels
gets larger as arteries divide into arterioles and capillaries, so the friction becomes greater.

loss of plasma as tissue fluid is formed in the capillaries.
As a result, the blood pressure falls and the blood has lost its pulsatile nature by the time it reaches
the venules.
8 (a) Heart rate = 60/0.8 = 75 beats minute−1.
(b) At P, the AV valve (bicuspid valve) is pushed closed because the blood pressure in the left
ventricle becomes greater than the blood pressure in the left atrium. At Q, the aortic valve is pushed
open because the blood pressure in the left ventricle becomes greater than the blood pressure in the
aorta. (When interpreting data questions like this, it might help to think of a door being pushed by two
people on opposite sides. Whoever pushes hardest determines whether the door is pushed open or
closed.)
Exam tip: You should use abbreviations, e.g. AV valve, only if they appear in your specification or are
given in an exam question. If not, you must write out the biological term in full, e.g. atrioventricular
valve.
9 The volume of blood he pumps out at each beat (the stroke volume) is greater than ‘normal’. The
total output of the heart (cardiac output) is:
cardiac output = heart rate × stroke volume
10 Impulses originate (genesis) directly from specialised muscle cells (myocytes) without nervous
stimulation.
11 It allows time for the atria to empty fully before the ventricles contract.
12 It enables the ventricle to empty fully. (Think of squeezing a tube of toothpaste from the middle;
toothpaste is pushed in both directions. Contraction of the ventricles from the middle would have a
similar effect.)
13 The cardiac cycle originates at the sinoatrial node, situated in the wall of the left atrium. From here,
impulses pass through the walls of the atria to the atrioventricular node and from there through the
septum in the bundle of His and up the walls of the ventricles via the Purkyne fibres. Although
originating in the heart itself, the rate of the cardiac cycle can be changed by the autonomic nervous
system. Release of noradrenaline by the sympathetic branch causes an increase in heart rate.
Release of acetylcholine by the parasympathetic branch causes a decrease in the heart rate.
14 A suitable mark scheme would cover the following six marking points.
Hydrostatic blood pressure is caused by contraction of left ventricle (1)
Hydrostatic pressure tends to force small molecules out of capillaries (1)
Water potential of blood (oncotic pressure if you follow the Edexcel specification) is caused by
plasma proteins (1)
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Water potential of blood (oncotic pressure if you follow the Edexcel specification) tends to pull
water into capillaries by osmosis (1)
Small gaps between cells allow water, ions and small molecules through capillary wall (1)
At arteriolar end of capillary, hydrostatic pressure  water potential so small molecules forced
out and form tissue fluid (1)
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